The present invention relates to a method of stably producing prepolymers of aromatic polythiazoles which can be converted by heating to rigid, high-strength aromatic polymers having excellent heat resistance, mechanical properties, chemical resistance, electric properties etc.
Conventionally, aromatic polythiazole has been produced from an aromatic dimercaptodiamine compound and a dicarboxylic acid derivative, particularly a chloride thereof. Because of its excellent mechanical strength, the aromatic polythiazole is expected to be used as a plastic material substituting for metal materials, alone or in combination with other engineering plastics.
In these circumstances, attempts have been proposed to provide polymer blend-type composite materials having so-called interstitial structure (molecular composite materials) by dispersing so-called rigid, high-strength aromatic polymers such as aromatic polythiazole in soft matrix polymers finely to the extent of molecular level. Since the unidirectional orientation of the molecular chains of rigid, high-strength polymers for increasing their strength is not performed in such molecular composite materials, the molecular composite materials show little anisotropy and excellent mechanical strength, heat resistance, solvent resistance etc.
However since the rigid, high-strength aromatic polymers such as aromatic polythiazole have extremely low dispersibility in matrixes of other polymers, it is practical matter difficult to produce the above molecular composite materials. In this circumstance, the present inventors previously noticed that prepolymers of rigid, high-strength aromatic polymers such as aromatic polythiazole are soluble in organic solvents together with matrix polymers such as nylons or their prepolymers, and found that by first blending the prepolymers of rigid, high-strength aromatic polymers with matrix polymers or their prepolymers in organic solvents and then heating the resulting blends to cause a ring closure reaction of the prepolymers of rigid, high-strength aromatic polymers, molecular composite materials containing uniformly dispersed rigid, high-strength aromatic polymers can be obtained. Based on this finding, the inventors previously filed patent applications for methods of producing such molecular composite materials in Japan (Japanese Patent Application Nos. 62-158631, 62-158632 and 62-172563).
In the meantime, the prepolymers of aromatic polythiazole have generally been produced by heating mixtures of aromatic diaminodithiol compounds or their salts and dicarboxylic acids or their derivatives in the presence of polyphosphoric acids, etc., at a temperature of 160.degree. C. or higher. However, it is difficult to control the polymerization reaction by heating to the above temperature in the presence of polyphosphoric acids. Thus, the polymerization reaction often proceeds to form polythiazoles. Therefore, various attempts have been made to provide the prepolymer of aromatic polythiazole.
Japanese Patent Laid-Open No. 60-223824 discloses a method of producing polythiazole prepolymers by reacting aromatic and/or heterocyclic diaminodithiol compounds with dicarboxylic acids in the presence of carbodiimide.
However, since the above reaction is carried out in the presence of carbodiimide, the actually formed polythiazole prepolymers are those having low molecular weights, suitable for photoresists, despite the description that the reaction proceeds at such a temperature that the prepolymers are not converted to inactive ring-closed products. The above low-molecular weight prepolymers are not suitable as prepolymers of high-strength polymers which are to be used in molecular composite materials.